In an enamel wire having an enamel layer baked on the outer periphery of a conductor, the enamel layer needs to have a thickness of 60 μm or more against partial discharge degradation. However, a problem occurs when the baking thickness per baking is set large, air bubbles are left in the enamel layer due to volatilization and bubble release in a solvent of varnish. On the other hand, when the baking thickness per baking is set small and the number of bakings is increased, the contact strength between the conductor and the enamel layer is deteriorated.
As a means to solve the above problems by increasing the coating thickness of an insulation layer and also enhancing joining strength of the enamel layer, Patent Document 1 proposes an inverter surge resistant insulated wire made by forming at least one enamel baking layer on an outer periphery of a conductor and forming at least one extruded resin coating layer on the outside thereof.
In Patent Document 1, thermoplastic resin is mainly used. Specifically, Patent Document 1 discloses that electric wires coated with various kinds of thermoplastic resin are produced and subjected to evaluations of abrasion resistance (room temperature), heat-resistance senescence characteristic (180° C.), and resistance to solvents as shown in Table 2.
On the other hand, Patent Document 2 discloses a method of manufacturing a stator by producing a cage-shaped insulated conductor coil by shaping an enamel flat rectangular wire having an enamel layer formed on an outer periphery of a flat conductor into straight in-slot conductor portions and coil end portions having bent portions, and then mounting the cage-shaped insulated conductor coil to a core.
In conventional stators, a voltage difference between the in-slot conductor portions located in adjacent slots is small and thus the enamel layer alone can provide sufficient insulation. In the coil end portion, however, three-phase currents are complicated, increasing a voltage difference between conductors, and thus the enamel layer alone cannot provide sufficient insulation. For this reason, an insulation interphase sheet is used to ensure insulation.
However, inserting the insulation interphase sheet between the coil end portions requires complicated operation, resulting in cost increase. To solve this problem, the use of the inverter surge resistant insulated wire has been investigated to ensure insulation in the coil ends and eliminate the use of the insulation interphase sheet.